In gamma-ray spectroscopy, measurement of the spectra of electromagnetic radiation emitted by a sample, and in particular the measurement of the energy distribution of gamma rays, is used to determine the radioisotopes of a sample. Gamma-ray spectrometers are used at nuclear power plants and at laboratories such as the Radiation Measurements Laboratory (RML) at the Idaho National Energy Laboratory (INEL) operated by EG&G Idaho, Inc. for the U.S. Department of Energy. The RML provides essential radioanalytical services to the Advanced Test Reactor (ATR), waste management programs and other projects. If monitoring requirements in the nuclear industry become more stringent in the future, more emphasis will be placed on the calculation of detection limits and threshold values from spectra.
Remote spectrometers have been fielded to provide monitoring of effluents during destructive nuclear fuel testing, as described by J. K. Hartwell, E. W. Killian, E. B. Shingleton, E. E. Owen and R. J. Norris in "An On-Line Automated Gamma Spectrometer for Coolant Monitoring at the Loss of Fluid Test Facility (LOFT)", IEEE Transactions on Nuclear Science, NS-30 (1983) and Jack K. Hartwell and E. Wayne Killian, "On-Line Gamma-Ray Data Acquisition at the Power Burst Facility", Nuclear Instruments and Methods in Physics Research, A242, p. 487-492 (1986). On-line gamma-ray spectrometers can be operated remotely to view and quantify the fission products released when reactor fuel test bundles sustain severe damage.
Gamma-ray spectrometric measurements can be complicated by factors such as pile-up and dead time, especially when the sample produces a high count rate. One solution for this problem includes injecting electronic pulses into the amplification circuitry through which the pulses from the gamma-ray detector are processed. This technique of electronic pulse injection has found wide application in Ge detector gamma-ray spectrometry for dead time and pulse pileup loss correction and for calculation or stabilization of the system gain and zero. However, in the standard implementation, careful attention is required to position the pulser peak(s) in an uncluttered spectral region. This is especially difficult when dual amplitude pulse injection is used and limits this application to sources in which the spectrum is known or can be previously measured. Additionally, the elevated continuum encountered at low energies in high count-rate spectra can obliterate the pulser peak from a low amplitude pulser when a moderate injection rate is used. A technique suggested by O. U. Anders in "Experience With the Ge(Li) Detector for High Resolution Gamma-Ray Spectrometry and a Practical Approach to the Pulse Pileup Problem", Nuclear Instruments and Methods, 68, pages 205-208, 1969 includes storage of pulser events in regions of the gamma-ray spectrum forbidden to gamma-ray events. A technique for pulser injection with subsequent removal that relies on software storage of pulser events in special buffers separate from the gamma-ray spectral storage region is described by L. 0. Johnson, E. W. Killian, R. G. Helmer and R. A. Coates, in "Utilization of Concurrently Gathered Pulser Data for Complete Spectral Validation of Gamma-Ray Spectra From Germanium Detectors", IEEE Transactions on Nuclear Science, NS-28 (Feb. 1981). Previous implementations of this latter technique, i.e. pulser injection with subsequent removal, have required custom-designed multi-channel analyzers (MCA) using a NOVA computer, as described by Johnson, et al., supra. This latter technique provides for real time spectral data validation, spectrometer performance monitoring, and spectrum specific energy calibrations without interference with the spectral data.
As an example, during the destructive fuel testing mentioned above, the pulser-based corrections for gain and zero shift and for correction for pulse pileup and system dead time were essential for accurate results. However, the complexity of the fresh fission product spectra accumulated during these experiments precluded storage of the pulser peaks within the gamma-ray spectral region. During a destructive fuel test more than 600 gamma-ray spectra were acquired. Input rates generally ranged from a few hundred counts per second to well over 100,000 c/s.
There are other popular loss-free or virtual pulser techniques, such as the virtual pulser technique described by G. P. Westphal in "Real-Time Corrections of Counting Losses in Nuclear Pulse Spectroscopy", Journal of Radioanalytical Chemistry, 70, p. 387, 1982. The virtual pulser technique would be expected to have the significant advantage of a nearly unlimited pulser sampling rate when the system dead time is very high and counting times are restricted. However, the virtual pulser technique does not provide the spectrum specific energy and width calibration available using the equally convenient pulser injection with subsequent removal method.
Therefore it is an object of the present invention to provide a gamma-ray spectroscopy system that includes pulse injection that separates the pulse and gamma-ray events.
It is a further object of this invention to provide a gamma-ray spectrometer that can separate the pulse and gamma-ray events automatically.
It is another object of this invention to provide a module the can be readily and efficiently adapted to existing gamma-ray spectroscopy system equipment to accomplish accurate separation of pulse and gamma-ray events in the memory of the multichannel analyzer.
It is still another object of the invention to provide an interface between a pulse injector and a multichannel analyzer that separates the pulser data from the gamma-ray data automatically using digital logic.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.